1. Field of the Invention
The present invention relates to a photoelectric conversion module which includes an optical element and a lead to pass through a package, and an optical transceiver using the same.
2. The Related Art of the Invention
FIG. 1 shows a conventional photoelectric conversion module 81. The conventional photoelectric conversion module 81 mounts a semiconductor laser (LD) element 82 on a heat sink (column) 84 inside a package 83 through a sub-mount 85.
The package 83 includes a disc-shaped stem 83s, and a cylindrical cap 83c to cover an upper surface of the stem 83s and the LD element 82. In general the package 83 is called “Can package” and used most widely as an inexpensive package of a photoelectric conversion module (LD module) for a container of the LD element 82.
The heat sink 84 is fixed on the stem 83s, and a plurality of substantially columnar leads 86 passing through the package 83 and made of metal of Fe group, are installed in the stem 83s. A lens 87 is mounted on an upper part of the cap 83c to collect a laser beam L from the LD element 82.
A cathode of the LD element 82 is electrically connected to the lead 86 by wire-bonding a gilded cathode electrode 88 formed on the surface of the LD element 82 to the lead 86 with a wire 89.
Further, an anode as a backside of the LD element 82 is soldered to the sub-mount 85 through a gilded anode electrode 90 formed on the sub-mount 85. The sub-mount 85 to which the LD element 82 is soldered, is fixed to the heat sink 84 with solder.
In case the stem 83s is an anode-type, which is not illustrated in detail, the anode of the LD element 82 is electrically connected to the heat sink 84 by a via hole to penetrate the anode electrode 90 and the sub-mount 85, or by wire-bonding the anode electrode 90 to the heat sink 84 with a wire. In the case of a floating-type, the anode of the LD element 82 is electrically connected to a different lead 86 by wire-bonding the anode electrode 90 to the different lead 86 with a wire.
The LD element 82 is so small that it is not easy to treat and locate it, and thus, as stated above, the LD element is mounted on the sub-mount 85 in advance in order to make the treatment easier, and then the sub-mount 85 gets installed on the heat sink 84.
The photoelectric conversion module 81 is used as built in an electronic device. The electronic device includes an LD driver for driving the LD element 82, and a substrate on which a semiconductor element such as the LD driver is installed.
Besides, since an optical communication is performed at a high transmission speed exceeding Gbps in recent years, an electronic device used for the optical communication, for instance an optical transceiver, may include a coaxial cable to transmit a high-frequency signal from the LD driver to the LD element 82.
It is noted that Japanese Unexamined Patent Publication No. 5-327617 exists as a technical document relating to the present invention.
However, in such photoelectric conversion module 81 it is not designed to make impedance matching between the LD element 82 in the package 83 and the components outside the package 83, such as the LD driver, the substrate, the coaxial cable, and the like.
In general the impedance of the LD element 82 (for instance, approximately 7 Ω) is different from that of the LD driver or the substrate (for instance, 25 Ω). As a result, in the module 81, when the LD element 82 emits light particularly at a high-frequency signal, the high-frequency signal causes reflection between the LD element 82 and the LD driver, or between the LD element 82 and the substrate, and thus the LD element 82 or the LD driver doesn't work accurately. Consequently the high-frequency characteristic is deteriorated.
The reflection of the high-frequency signal is also produced between the LD element 82 and the coaxial cable (for instance, impedance 50 Ω), and therefore, there occurs also a problem that the high-frequency characteristic of the photoelectric conversion module 81 is deteriorated.
In order for the impedance of the LD element 82 to match that of those components outside the package-83, there are the following two methods to be considered.
A first method achieves the impedance matching by disposing a terminal resistor to be connected with the LD element 82 outside the package 83. However, since in the first method, the distance between the LD element 82 and the terminal resistor tends to be long, there is a case where reflection of the high-frequency signal between the LD element 82 and the terminal resistor can be produced. That still causes a problem of deteriorating the high-frequency characteristic of the module 81. That is, unless the terminal resistor is set adjacent to the LD element 82, the impedance-matching is not performed efficiently.
A second method achieves the impedance matching by receiving a terminal resistor to be connected with the LD element 82 inside the package 83. However, since in the second method, the package 83 is so small, it is difficult to receive a new terminal resistor inside the package 83 from the spatial point of view. Accordingly if the new terminal resistor is installed inside the package 83, the photoelectric conversion module 81 will become bigger in size. In addition, in this case heat generated by the terminal resistor can be another problem.